Dispersions based on precipitated silicon dioxide have already been described in the prior art. A core theme in the prior art publications is the stabilization of the dispersions.
For example, Japanese published specification JP09142827 describes stable silica dispersions, the storage stability of which is achieved by virtue of the mean particle size of the silica particles being less than 100 nm. These dispersions have the disadvantage that it is very complicated and energy-intensive to grind silica particles to such small particle sizes. The process described in JP0914287 is therefore unlikely to gain any technological relevance for economic reasons.
EP 0368722, EP 0329509, EP 0886628 and EP 0435936 describe dispersions of silicas stabilized by means of stabilizers. One reason for adding the stabilizers is to prevent the sedimentation of the particles. The stabilizers are, for example, biogum or a system composed of aluminium compounds and anionic dispersants, or latex, or finely divided solids, which are chemically and physically compatible with the silicon dioxide. The use of such stabilizers is disadvantageous both for reasons of cost and with regard to the later use of the dispersions. More particularly, such dispersions are unsuitable for production of topcoats for high-gloss photo paper for inkjet printing.
Another approach to the stabilization of silicon dioxide dispersions is presented in DE102006049526.8. This describes dispersions of precipitated silica which are storage-stable owing to an alkaline pH and a strongly negative zeta potential. These are thus anionically stabilized dispersions. However, these dispersions are unsuitable for production of topcoats for high-gloss photo paper for inkjet printing, since the usually anionic dyes have to be fixed on a cationic porous print carrier, in order to achieve water resistance and high brightness among other properties.
Cationically stabilized dispersions are likewise already known. For instance, DE-A-10033054 describes the stabilization of a silica dispersion with the aid of cationic organic polymers. U.S. Pat. No. 6,777,039 describes the production of a coating for an inkjet printing medium by adding an aqueous solution of polyvinyl alcohol, an organic solvent and a surfactant to a dispersion of silica and a cationic polymer. U.S. Pat. No. 6,417,264 describes a dispersion of silica, which has been dispersed with an organic cationic polymer in a polar solvent. U.S. Pat. No. 6,420,039 describes a cationic silica dispersion in which SiO2 particles are bonded to an aluminium compound in order to achieve stabilization.
Stabilization with cationic polymers, for example p-DADMAC, is of interest in particular for dispersions of fumed silicon dioxides. For dispersions of precipitated silicas, however, this method has been found to be problematic since polyvalent anions from the precipitation process, for example sulphate ions, led to the reagglomeration of the dispersed silica particles. Therefore, the methods mentioned have insufficient suitability for the cationic stabilization of precipitated silica dispersions.
An alternative approach to the cationic stabilization of silica dispersions is given by EP 1 894 888 A1. This discloses that direct modification of silica with aminosilanes gave, in a simple manner, highly stable dispersions with a minimum of additional reagents. However, the dispersions obtainable by this process, according to the examples, have solids contents of only 10 to 19% by weight, which appears unsuitable for a commercial application.
Owing to the significantly lower production costs for precipitated silicas compared to fumed silicas, there is still a need for a process which allows inexpensive production of cationically stabilized precipitated silica dispersions with high solids content and good storage stability. Ideally, this process should allow the stabilization of the precipitated silica dispersion even in the presence of polyvalent anions or significant amounts of inorganic salts in the dispersion, such that it is unnecessary to fully remove these salts/anions in complex purification steps.